In the past few years remarkable progress has been made in the development of high power density AC/DC converters using resonant-link schemes which utilize high speed devices such as fast recovery transistors and GTO's. These new converters not only have high power density but also possess very low switching losses because switching of the devices takes place at zero-voltage instants and thus the total system is able to operate at very high frequency compared to conventional DC link transistorized converters. Although these resonant-link converters are intended to operate at high power density, almost all the systems require self commutated switching devices and have some difficulty performing conversion at very high power levels because of the relatively low voltage and current margins that self commutated devices, such as transistors, typically have.
In general, switching schemes for resonant converters can be classified according to their resonant AC-link and resonant DC-link modes. The resonant AC circuits utilize either a parallel resonant circuit (FIG. 1) or a series resonant circuit (FIG. 2). The AC resonant circuit impresses both polarities of AC voltage and current on the link so that the switches of the input and output side converters are required to carry both positive and negative currents as well as block both polarities of voltage. The converter switches must therefore be bi-directional switches which are usually realized by two inverse-parallel transistors or thyristors for the parallel and series resonance circuits, respectively.
DC-link circuits have been developed which realize pulsating DC currents in the link by adding DC offsets to the AC resonant current. The resonant DC-link converters reported in the past have been parallel resonant types, as shown in FIG. 3. See also, U.S. Pat. No. 4,730,242 to Divan, which discloses parallel resonant link converter systems wherein the switching devices are switched at times of zero voltage to minimize switching losses.